It is known to produce fluorinated compounds, such as hydrofluorocarbons, by fluorination of chlorinated compounds, such as hydrochlorocarbons. This fluorination is generally a catalytic fluorination using hydrofluoric acid (HF) as fluorinating agent. During this type of reaction, hydrochloric acid (HCl) is coproduced. In practice, it is furthermore not always possible or desirable to react all of the HF involved. The latter is then re-encountered in the form of an impurity associated with the hydrochloric acid. Other reactions for the synthesis of fluorinated compounds also result in the coproduction of HCl contaminated by a small amount of HF. Such is in particular the case of the manufacture of vinylidene fluoride by pyrolysis of 1-chloro-1,1-difluoroethane.
Insofar as hydrochloric acid cannot be discharged to the environment, techniques have been provided to recover it in value. It is thus known to separate the HCl from the other gases produced (including the fluorinated compounds of interest) by adiabatic absorption in order to generate an HCl solution of commercial type and to recover, at the top, the crude gases to be treated. However, in the case where hydrochloric acid is coproduced in a pyrolysis process, it has been observed that the pyrolysis could result in the formation of tars liable to foul the adiabatic column. In addition, the HCl solution thus obtained is contaminated not only by small amounts of hydrofluoric acid (HF) but also by organic pyrolysis residues and organo-halogen compounds which constitute byproducts of the pyrolysis. This HCl solution is thus not sufficiently pure for the majority of applications.
Processes have been provided for improving the purity of an HCl solution contaminated by HF and optionally halogenated organic compounds.
Thus, the document FR 1 507 252 describes a process targeted at separating hydrofluoric acid mixed with HCl, which consists in passing this gaseous mixture into a continuous countercurrentwise washing device comprising a plate column, the washing being carried out using a concentrated aqueous hydrochloric acid solution at low temperature which is capable of absorbing the hydrofluoric acid. There are obtained, at the outlet of this column, an aqueous solution of hydrochloric acid and hydrofluoric acid, and also gaseous hydrochloric acid, depleted in HF with respect to the starting hydrochloric acid. However, in order to obtain hydrochloric acid virtually devoid of HF, it is necessary to pass the crude gas mixture over active carbon in the presence of water, upstream of the acid washing stage. It is understood that this process is thus not applicable to a crude gas mixture including a fluorinated gas of interest, which would then be trapped in the active carbon.
Furthermore, it has been suggested, in the document WO 2015/079137, that the hydrochloric acid coproduced in a catalytic fluorination process could be purified following a process involving a stage of catalytic hydrolysis of the fluorinated/oxygenated compounds present in the crude gas stream, followed by a stage of washing with an acid solution, then by a stage of adsorption of the gases on an active carbon bed and, finally, by a stage of adiabatic adsorption, making it possible to obtain an aqueous hydrochloric acid solution. This solution can optionally be subsequently purified over silica gel. This process necessarily incorporates a preliminary stage of distillation of the crude gases in order to prevent the fluorinated product of interest from being trapped in the active carbon bed employed in the catalytic hydrolysis stage. It would thus be desirable to have available a process which is not more complex than that described in the application WO 2015/079137 but which makes it possible to directly treat the crude gas stream resulting from the catalytic reaction, without it being necessary to subject it to a prior distillation.